For any woman undergoing in vitro fertilization (IVF) to overcome infertility, the big question is: What are my chances of having a baby? Pretty good, according to a new study, which finds that women's chances of live birth via IVF may be similar to those of other women their age in the general population  better for younger women, not as good for older ones.

In an analysis of the overall live-birth rate for 6,164 patients at a Massachusetts fertility clinic, researchers from Boston IVF and Beth Israel Deaconness Medical Center found that in women ages 39 and younger who were treated with up to six cycles of IVF, the rate of live births ranged from 65% to 86%; in women ages 40 and older, the live-birth rate was 23% to 42%.

"It's fantastic," says Dr. Beth Malizia, lead author of the study, published in the Jan. 15 issue of the New England Journal of Medicine, and a former clinical fellow in obstetrics, gynecology and reproductive biology at Harvard Medical School. "When we got these results we were quite pleased...[IVF] does restore in the infertile population the same live birth rates that they have in their own age group."(See TIME's Top 10 Medical Breakthroughs of 2008)

Not all of the study's more than 6,000 participants became pregnant and gave birth, or underwent the full six cycles of IVF before leaving the clinic, which accounts for the range of live-birth rates. Researchers had to extrapolate from their existing data the likely outcomes for patients who discontinued treatment: In the "optimistic" analysis, researchers assumed that the women who discontinued treatment would have had the same success rates as those who continued treatment; the "conservative" analysis assumes a zero success rate among all women who discontinued IVF at that particular clinic. Realistically, say the authors, the actual live-birth rates would fall somewhere in the middle of the range.

Another possible limitation of the study, the authors say, is the fact that all patients seeking infertility treatment in the state of Massachusetts are at least partially reimbursed by their health-insurance carriers, by law. IVF can cost up to $10,000 or more per cycle, and the study group at the Massachusetts clinic likely included many patients less deterred by financial issues than women in the general population, possibly distorting the data. Malizia, who now serves as a reproductive endocrinologist at Alabama Fertility Specialists in Birmingham, Ala., suggests, however, that the live-birth rates of her study group skewed lower than they might have, had the study been conducted elsewhere. "In states where there's no insurance coverage, there's lower access to IVF treatment," she says. "Those who pursue it may have a better chance of pregnancy, so our suspicion would be that the cumulative rates would be higher in states without insurance." (Read "Predicting In Vitro Success")

Malizia's study may not break new medical ground, but it presents a new perspective on IVF, in that most studies that analyze the treatment's success rates look solely at the outcomes of individual implantations, not the overall possibility of a live birth. "As far as the patients are concerned, they would like to know not just pregnancy but live birth [outcomes], and not just during that one cycle but over the course of the entire treatment," Malizia says. "The statistics that are out there don't exactly answer the question."

The Centers for Disease Control and Prevention collects national statistics on success rates for assisted reproductive technology, but that data, too, is broken down by individual cycles of IVF; the data in the new study calculated the overall likelihood of a live birth during the entire course of treatment  up to six cycles of IVF. The study also incorporated data for all patients who sought treatment at the Massachusetts center, regardless of age or potential for pregnancy complications. In addition, the data accounted for success rates of IVF using frozen embryos; other analyses typically consider only "fresh" embryo use.

For Dr. Mitchell Rosen, director of the Fertility Preservation Center and Reproductive Laboratories at the University of California at San Francisco, the inclusion of frozen-embryo data is notable. "The most unique part of the study is that frozen embryo transfers were considered unique treatments," he says. But Rosen says he would have liked to see the analysis go a step further, detailing which cycles involved frozen embryos and how long embryos were in culture before they were frozen, for example. (The longer embryos are in culture the fewer survive, but due to the selection process the odds of implantation are higher.)

From a counseling point of view, says Rosen, this type of study may offer a valuable model for other centers, and could lead to additional center-specific studies that offer more insight for infertility patients. "It's an interesting, creative way of presenting the data because you're getting a more accurate representation of the general population that is going through assisted reproductive technology, and of what their success rates are going to be to take home a baby."

For Malizia, that was precisely the goal. "My hope would be that physicians around the country would use this information in counseling patients," she says, in turn arming those patients with more information to make wiser decisions about treatment.